In recent years, with the spread of PCs (Personal Computers) equipped with AV (Audio Visual) functions or video/audio communication through Internet, a demand for high-capacity and high-speed disk type recording media for recording and reproducing video/audio data has increased.
To meet the demand, various kinds of recording and reproduction devices employing disk type recording media, such as magnetic disks and optical disks, have been proposed. Especially, rewritable optical disk units that are removable using cartridges (hereinafter, referred to simply as `optical disks`), such as optical magnetic disks and phase changeable optical disks, have spread rapidly because these disks are easy to handle and have large capacities.
However, compared with hard disk units employing magnetic disks (hereinafter, referred to simply as `hard disks`), optical disks are, by one figure or more, lower in seek time and data transfer speed than the hard disks. Hence, optical disks are likely to be used restrictively for purposes of relatively low access frequency, such as backup.
On the other hand, moving picture compression techniques of MPEG (Moving Picture Experts Group) system make a rapid transition from MPEG1 to MPEG2. With this transition, required bit rate increases significantly, for example, the bit rate required for MPEG2 DVD (Digital Versatile Disc) is 3.about.11 Mbps while the bit rate required for MPEG1 video CD is only 1.5 Mbps. Therefore, in authoring units for business use, expensive disks like the newest high-speed disks or RAID (Redundant Array of Inexpensive Disks) are employed. Further, large-capacity and high-speed hard disks for PCs and the like have been proposed in recent years. However, with the MPEG2 class data rate, it is difficult to secure real-time recording.
As described above, hard disks and optical disks are required to have higher processing speed for recording/reproduction of real-time continuous data such as video data and audio data.
In recent disk type recording media, to increase memory capacity and recording density, adopted is a method of dividing a disk into plural sub-areas and increasing the number of sectors per track in each area from the inner circumference toward the outer circumference, represented by a Z-CAV (Zone-Constant Angular Velocity) method. For example, according to the standard of a phase changeable optical disk, this disk is divided into ten areas, and an area at the innermost circumference of the disk has 30 sectors per track. The sector number increases by 4 sectors per track in each area toward the outer circumference. An area at the outermost circumference has 66 sectors per track, that is, 36 sectors per track are increased as compared with the innermost circumference. The disk is rotated at a constant rotation speed, and the frequency of recording/reproduction is changed area to area. That is, in this standard, the recording/reproduction rate at the outermost circumference is more than twice the recording/reproduction rate at the innermost circumference (by simple calculation, 66/30=2.2 times).
As described above, in the disk type recording media, the recording/reproduction rate, i.e., the recordable/reproducible data quantity per unit time varies area to area and, generally, it is higher at the outer circumference than at the inner circumference. However, in an application in which continuous data is recorded and reproduced, like MPEG, real-time recording/reproduction of data of a predetermined quantity per unit time is required. Therefore, when disk type recording media are used, the recording rate (the quantity of data to be recorded in the unit time) is set to that of an area of the lowest speed, or only areas of recording-rate performances (the quantities of data recordable in the unit time) exceeding the recording rate are used. Consequently, in the existing circumstances, efficient use of disk media is impossible.
By the way, the smallest unit of a recording area managed by a disk unit is called `sector`, and a sector is 512 bytes in a hard disk. It is generally known that, in a disk unit, higher-speed recording/reproduction is realized by recording/reproducing a larger quantity of data at one time.
For example, in an operating system (OS) for PC, a file allocation size, which is the smallest unit of area management in a disk unit having a capacity of 500 MB.about.1 GB, is about 16 kB. However, if circumstances permit, actual recording/reproduction is carried out with a file allocation size of 32 kB.about.64 kB. To increase the file allocation size, a method called `delay writing` is used for writing of data in the disk unit, whereby a high-speed file system is implemented. In this method, when a request to write data in the disk unit (hereinafter, referred to simply as request-to-write) is given, data to be written is temporarily stored in a write buffer provided on a main memory and, without waiting for completion of actual writing of data in the disk unit, the request-to-write is completed. Thereby, while the write buffer on the main memory functions effectively, the request-to-write can be ended in a moment, viewed from the application program.
However, when the write buffer on the main memory is filled up, requests-to-write after that are stood by unit a capacity corresponding to the data quantity requested by the request-to-write is secured in the write buffer. At this time, the data in the write buffer decrease at the recording rate of data into the disk unit, if the writing in the disk unit is carried out on the area of the lowest rate, the request-to-write is processed at a recording rate equal to the lowest rate. Accordingly, in case of processing continuous data like video and audio data, when data is generated at a rate (quantity of data per unit time) larger than the recording-rate performance of the lowest-speed area in the disk, the write buffer on the main memory will be filled up soon. Depending on the area on which data is being written, there is a risk that the data generating process is ended in the middle or a portion of generated data is lost.
Further, in a standard file system of PC, it is impossible to perform recording by designating an area of a recording medium from an application program. So, in order to use only high-speed areas, low-speed areas must be isolated, leading to a reduction in the capacity.